Ocean Acidification and the Humboldt Squid

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“While many other squid and octopus species have
oxygen transport systems that are equally
sensitive to pH, few have such high oxygen demand
coupled with large body size and low
environmental oxygen. Therefore the scientists
believe that their study results should not be
extrapolated to other marine animals.”
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KINGSTON, R.I. – December 15, 2008 – The elevated
carbon dioxide levels expected to be found in the
world’s oceans by 2100 will likely lead to
physiological impairments of jumbo (or Humboldt)
squid, according to research by two University of
Rhode Island scientists.

The results of a study by Brad Seibel, URI
assistant professor of biological sciences, and
Rui Rosa, a former URI post-doctoral student now
on the faculty at the University of Lisbon,
Portugal, is reported in this week’s issue of the
Proceedings of the National Academy of Sciences.

The researchers subjected the squids (Dosidicus
gigas) to elevated concentrations of CO2
equivalent to those likely to be found in the
oceans in 100 years due to anthropogenic
emissions. They found that the squid’s routine
oxygen consumption rate was reduced under these
conditions, and their activity levels declined,
presumably enough to have an effect on their
feeding behavior.

Jumbo squid are an important predator in the
eastern Pacific Ocean, and they are a large
component of the diet of marine mammals, seabirds
and fish.

According to Seibel, jumbo squid migrate between
warm surface waters at night where CO2 levels are
increasing and deeper waters during the daytime
where oxygen levels are extremely low.

“Squids suppress their metabolism during their
daytime foray into hypoxia, but they recover in
well-oxygenated surface waters at night,” he
said. “If this low oxygen layer expands into
shallower waters, the squids will be forced to
retreat to even shallower depths to recover.
However, warming temperatures and increasing CO2
levels may prevent this. The band of habitable
depths during the night may become too narrow.”

Carbon dioxide enters the ocean via passive
diffusion from the atmosphere in a process called
ocean acidification. This phenomenon has received
considerable attention in recent years for its
effects on calcifying organisms, such as corals
and shelled mollusks, but the study by Seibel and
Rosa is one of the first to show a direct
physiological effect in a non-calcifying species.

The scientists speculate that the squids may
eventually migrate to more northern climes where
lower temperatures would reduce oxygen demand and
relieve them from CO2 and oxygen stress. While it
is possible, they say, that the squids could
adjust their physiology over time to accommodate
the changing environment, jumbo squids have among
the highest oxygen demands of any animal on the
planet and are thus fairly constrained in how
they can respond.

“We believe it is the blood that is sensitive to
high CO2 and low pH,” Seibel said. “This
sensitivity allows the squids to off-load oxygen
more effectively to muscle tissues, but would
prevent the squid from acquiring oxygen across
the gills from seawater that is high in CO2.”

While many other squid and octopus species have
oxygen transport systems that are equally
sensitive to pH, few have such high oxygen demand
coupled with large body size and low
environmental oxygen. Therefore the scientists
believe that their study results should not be
extrapolated to other marine animals.